Coatings for high-temperature alloys

ABSTRACT

COLUMBIUM OR ITS ALLOY HAVING A TITANIUM-MODIFIED COLUMBIUM SILICIDE COATING HAS AN OVERCOATING OF CHROMIUM ON THE SILICIDE. THE COATING HAS HIGH RESISTANCE TO THERMAL AND MECHANICAL SHOCK FAILURE. THE SILICIDE COATED COLUMBIUM MAY BE COATED WITH CHROMIUM BY SURROUNDING THE COATED METAL WITH A POWDERED PACK OF CHROMIUM AND A VOLATILIZABLE HALIDE AND AN ENERT FILLER AND HEATING TO CAUSE VOLATILIZATION OF THE HALOGEN IN THE HALIDE SALT.

COATINGS FOR HiGH-TEMPERATURE ALLOYS Filed April 14. 1964 2 Sheets-Sheet 1 Cr Overcoat L Ti-Modified CbSiZ Incipient Diff usio n Zone Cb-lZr Substrate FIG. I. I 0 4 x Cr Overcouied Ti-Modified Coiumbium Silicide Coating on Cb-lZr 500x Substrate (As Coated Condition) Oxide Zone (Oxidic Cr) Cr Overcout Ti-Modified CbSi Diffusion Zone (Subsiiicides of 1 v w Substrate) 0 Q FIG. 2.

Cr Overcooted Ti-Modified Columbium Silicide Cogting on Cb-lZr Subsirute After Exposure in Static An 0 LEONARD A. FRlEDRICH for 1000 Hours 0? I800 F EMANUEL C. HIRAKIS 1111727, 1971 L. A. FRIEDRICH ETAL 3,595,533

COATINGS FOR HIGH-TEMPERATURE ALLOYS Filed Agril 14. 1964 2 Sheets-Sheet 2 LEONARD A. FRIEDRICH BY EMANUEL C. HIRAKiS .4 TTOR NE w United States Patent 3,595,633 COATINGS FOR HIGH-TEMPERATURE ALLOYS Leonard A. Friedrich, West Hartford, and Emanuel C.

Hirakis, Mansfield Center, Conn., assignors to United Aircraft Corporation, East Hartford, Conn.

Filed Apr. 14, 1964, Ser. No. 360,178

Int. Cl. B32p 3/00 US. Cl. 29-198 25 Claims ABSTRACT OF THE DISCLOSURE Columbium or its alloy having a titanium-modified columbium silicide coating has an overcoating of chromium on the silicide. The coating has high resistance to thermal and mechanical shock failure. The silicide coated columbium may be coated with chromium by surrounding the coated metal with a powdered pack of chromlnm and a volatilizable halide salt and an inert filler and heating to cause volatilization of the halogen in the halide salt.

This invention relates to novel coatings for columbium and columbium base alloys that will protect the base metal or alloy from oxidation at high temperatures and to a method for creating such coatings.

More particularly, this invention relates to chromium overcoatings on base coatings of titanium-modified columbium silicide or of molybdenum-titanium-modified columbium silicide, to form composite coatings for columbium and its alloys in which the coatings are created by methods, such as, vapor deposition, electrophoretic deposition, and t e like. The invention also particularly relates to a method for vapor depositing such chromium overcoatings on base coatings of titanium-modified silicide or of molybdenum-titanium-rnodified silicide, to form composite coatings on columbium base materials that produce a protective layer or zone over such materials to provide an oxidation-resistant coating at high temperatures, such as, for example, temperatures up to at least 2000 F. in air.

During exposure to an oxidizing environment at elevated temperatures an oxide coat is formed on the outer or exposed zone of the coating and at diffusing temperatures an inner diffusion zone consisting essentially of subsilicides of the substrate is created between the Timodified or Mo-Ti-modified columbium silicide and the substrate.

For many years it has been generally known that the high-temperature strength properties of metals are closely related to their melting points. In general, metals having high melting points thus are capable of forming alloys having high strength at high temperatures.

The need for structural materials for service at temperatures in excess of those obtainable with existing structural materials has stimulated interest in the metals having the highest melting points, or the refractory metals, particularly, chromium, columbium, tantalum, molybdenum and tungsten.

Molybdenum was once considered the chief prospect as a base metal in alloys for such usage. At the hightemperature service conditions needed, however, molybdenum not only oxidizes, but the molybdenum oxide formed is volatile at these elevated temperatures, and accordingly, once the oxidation reaction begins it tends to progress rapidly until molybdenum is consumed at a catastrophic rate.

As an alloy base material for high temperature service, columbium offers more promise, and considerable interest has been directed to its use as a structural alloy base for applications in high-temperature environments. Among the technically most important physical qualities ice of columbium as an alloy base are its high melting temperature (4474 F.) and its low neutron-capture crosssection. Columbium is, therefore, potentially useful as a structural material for containment vessels for high-temperature liquid metals.

Further, columbium is inherently a soft, ductible, readily fabricable material, and although it becomes too weak for practical structural uses at temperatures much above 1200" F., it readily can be strengthened for use at much higher temperatures by alloying with various metals, and particularly by alloying with the refractory metals. Columbium is also a highly reactive metal in that it dissolves large quantities of oxygen and nitrogen, upon exposure to atmospheres containing even small amounts of these elements at relatively modest temperatures.

Although columbium oxidizes rapidly at high temperatures, in contrast to molybdenum which oxidizes catastrophically, columbium oxide does not volatilize. It is thus potentially possible to prevent oxygen attack on columbium by coating the metal, and if premature localized coating failure should occur, to restrict such failure and oxygen attack to the localized site. Further advantages offered by columbium over molybdenum base alloys are that columbium base alloys are relatively more ductible and workable at low temperatures and columbium has a lower density than molybdenum.

The history of columbium alloy technology has, however, demonstrated the incompatibility of achieving oxidation resistance and high-temperature strength through alloying alone. Since the major uses for columbium base alloys are as structural components in high-temperature applications, it is apparent that useful classes of hightemperature columbium alloys will require protective coatmgs in their normal high-temperature oxidizing environments.

A particularly important potential area of use for columbium base alloys as dictated by economic and technological factors is in uses of such alloys requiring exposure to oxidizing environments at temperatures up to about 2000 F. (a temperature that clearly establishes utility for these alloys). Concomitantly, such alloys must be able to resist mechanical stresses for appreciable periods of time at these high temperatures.

About 500 F. is the maximum operating temperature to which columbium base alloys may be subjected for extended times in the uncoated condition without serious oxidation, and at temperatures much above 500 F. the oxidation problem becomes acute.

The art has previously recognized certain oxidation resistant intermetallic coatings as exhibiting particular potential for protecting refractory metals (e.g., columbium, molybdenum, tantalum, and tungsten) from oxidation at high temperatures. In general, the more effective of these intermetallic coatings are silicides, aluminides and beryllides of the base metal.

In considering coatings for the refractory metals, both coating and substrate materials importantly affect the performance of the coated systems. For example, a silicide coating over columbium may perform quite differently from one over molybdenum with the difference in performance attributable to the substrate rather than to the coating type. As an additional confirmation of the importance of the substrate, some species of coatings that are reliably protective over other of the refractory metals are ineffective over columbium and because they are susceptible to failure on columbium at high temperatures. Coating and substrate must thus be evaluated and treated as an integrated system. Success with a particular coating on a particular base metal does not mean the coating will be successful when used over a different base metal.

Several methods, such as, flame or plasma torch spraying, slurry application techniques, electrophoretic deposition, hot pressure bonding or vapor deposition, have been used for applying intermetallic coatings to columbium base alloys. A vapor deposition process that can be used advantageously to achieve some types of coatings is the so-called pack-cementation process in which the object to be coated is surrounded by a particulate pack mixture containing, for example, (1) the metal to be reacted with or deposited upon the object to be coated (e.g., silicon, aluminum, beryllium), (2) an activator or energizer (usually a halide salt, such as, NaCl, KP, NH I, NH Cl, and the like), and (3) an inert filler material (e.g., A1 SiO BeO, MgO, and the like).

This mixture, held in a suitable container (such as, a steel box, graphite boat or refractory oxide crucible), is then heated to the desired coating temperature in a prescribed atmosphere and held for a length of time sufiicient to achieve the desired coating. When conducted properly, the pack-cementation process may be used to produce controlled-thickness coatings on columbium, the major proportions of which may be compounds, such as, CbA1 CbSi and the like.

The more favorable coatings for columbium (columbium aluminides, silicides, beryllides) possess certain intrinsic deficiencies such as rapid oxidation failure at low temperatures (in the vicinity of 1300 F.) or at high temperatures (about 2000 F. and above). Perhaps the most serious deficiency of existing coatings for columbium, however, is their propensity for failing at localized sites.

Silicide coatings on columbium and its structural alloys are more stable than aluminides and have a better thermal expansion match with the substrate than beryllides which have such a severe thermal expansion mismatch that it prohibits their use. With columbium the silicides are thus of primary interest.

Silicide coatings on structural columbium alloy substrates, however, are prone to consumption by rapid oxidation at low (about 1300 F.) temperatures. (This characteristic of silicide coatings is sometimes termed the silicide pest phenomenon) Modification of silicide coatings is thus highly desirable to impart sufiicient longevity and reliability to give to them a utility they do not normally possess.

Copending applications Ser. No. 360,176, filed Apr. 14, 1964, and Ser. No. 360,177, filed Apr. 14, 1964, disclose a titanium-modified columbium silicide coating composition and a molybdenum-titanium-modified silicide coating composition that effectively protect columbiumbase alloys from oxidation in static air at temperatures up to at least 1800 F. for times in excess of 5000 hours.

Coatings of these compositions are particularly effective in overcoming the silicide pest phenomenon, characterized by consumption of columbium silicide coatings by rapid oxidation at temperatures of about 1300 F., and also in overcoming the rapid oxidation of columbium silicide coatings at higher temperatures of about 2000 F. It has been discovered, however, that although such coatings are distinctly more reliable than the coatings of the prior art, their reliability, and in particular their resistance to the silicide pest phenomenon, can be significantly improved by the addition of a pellicular chromium overcoat to the base coatings.

Accordingly, it is a primary object of this invention to provide a new and improved coating for columbium and its alloys comprising a titanium-modified or molybdenumtitanium-modified columbium silicide base coating composition having a pellicular chromium overcoat. This chromium overcoat on the Ti-modified and Mo-Ti-modified columbium-silicide coatings importantly improves these base coatings by achieving the new and useful result of completely getting rid of the silicide pest problem and eliminating fissuring as a failure mechanism. The

4 reliability, resistance to failure, and life of the base coatings are thereby significantly increased.

Although not fully understood, it is believed that the chromium in the overcoat serves to seal or plug off fissures in the coatings once these start, but regardless of the correctness of the theory of operation, it has been established that the chromium overcoat works exceptionally well in preventing failure by fissuring, and the new and useful result of the invention is not restricted to any particular theory of operation.

Another object of this invention is to provide, as a composite coating for columbium and its alloys, chromium overcoats on Ti-modified and Mo-Ti-modified columbium silicide base coatings over columbium substrates. Such composite coatings in addition to providing resistance to simple thermal oxidation are also protective under other reasonably expected conditions of use, and to this end the protective coatings of this invention achieve good resistance to thermal cycling, thermal shock, and formation of defects. They are also diffusionally stable, well-bonded to the substrate and resistant to spalling.

Other objects of this invention are to provide for columbium and its alloys:

(1) A coating that in nominal thicknesses of 1 /2 to 3 mils, including a chromium overcoat of less than 1 mil in thickness, is capable of providing protection to exposures in static air for times in excess of 5000 hours at temperatures up to at least 1800 F.;

(2) A coating that exhibits excellent resistance to thermal and mechanical shock failure;

(3) A coating that displays excellent resistance to the formation of defects at both higher and lower tempera" tures of exposure; and

(4) A coating that gets rid of the silicide pest phenomenon and eliminates fissuring failures.

A still further object of this invention is to provide a method of coating columbium and its alloys with Tirmodified and Mo-Ti-modified columbium silicide composition coatings having pellicular chromium overcoats by a two-step vapor deposition (pack-cementation) process that achieves substantial uniformity of the coating and yields an essentially uniform coating on even intricately shaped parts and at the edges and corners of parts.

Additional objects and advantages of the invention will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention, the objects and advantages being realized and attained by means of the compositions, methods and processes particularly pointed out in the appended claims.

The term diffusing temperature refers to those temperatures at which a diffusion zone forms by interdiffusion between the coating and substrate taking place at an ap preciable rate. A preferred diffusing temperature for the coatings of this invention is 1800 F. At this temperature interdiffusion proceeds at an advantageous speed until a distinct diffusion zone is formed, and its is unnecessary and uneconomical to use a higher temperature. Once the diffusion zone is formed it essentially stabilizes, and loss of the coating through diffusion into the substrate is prevented.

Diffusing temperatures of from 1700 to 1900 F. are efiicacious, and even higher temperatures up to 2300 F. would achieve the desired result, but at 2300 E, if the diffusion is accomplished in an oxidizing atmosphere, the life of the coated article would be short.

Exposure to the diffusing temperature is normally effected during actual use of the coated articles and not as a separate step, and is thus simultaneous with exposure to an oxidizing environment at elevated temperatures. At about 1800 F. diffusion begins with exposure and continues until the structure of the coating system becomes essentially stabilized after about 25 hours of exposure.

To achieve the foregoing objects and in accordance with its purpose, this invention includes an article of manufacture having good resistance to oxidation in air at elevated temperatures, which article comprises a core of metal selected from the group consisting of columbium and columbium base alloys, the article having a defect, spalling, fissuring and thermal and mechanical shock failure resistant coating having a surface zone consisting essentially of chromium and a subsurface or inner zone consisting essentially of columbium silicide (preferably CbSi modified by titanium, the coating being characterized by great resistance to fissuring failure.

The invention also includes in all its various herein described embodiments such an article in which the titaniummodified columbium silicide subsurface or inner zone of the coating is further modified by molybdenum.

The invention further comprehends an article of manufacture having good resistance to oxidation in air at elevated temperatures, which article comprises a core of metal selected from the group consisting of columbium and columbium base alloys, the article having a defect, spalling, and thermal and mechanical shock failure resistant shell or coating, the shell or coating consisting essentially of a pellicular surface zone of chromium, a subsurface zone of columbium silicide (preferably CbSi modified by titanium, and, after exposure to a diffusing temperature in a nonoxidizing atmosphere, a diffusion zone beneath the subsurface zone consisting of subsilicides of the metal core.

The invention may also be described as including a new and improved article of manufacture having good resistance to oxidation in air at high temperatures, which article comprises a core of metal selected from the group consisting of columbium and alloys thereof, the article having a thermal and mechanical shock failure resistant, defect resistant, spalling resistant, fissuring resistant, and broad range oxidation resistant shell or coating, this shell or coating being characterized after exposure to an oxidizing environment at a temperature effective to create diffusion be tween the coating and metal more by an exposed layer or surface zone consisting essentially of oxidic chromium, a pellicular subsurface layer or zone consisting essentially of chromium, an intermediate layer or second subsurface zone consisting essentially of columbium silicide (preferably CbSl2) modified by titanium, and an inner layer or diffusion zone beneath the second subsurface zone consisting essentially of subsilicides of the metal core, the shell or coating being characterized by great resistance to failure by fissuring and by freedom from the silicide pest problem.

This invention also embraces as an article of manufac ture, a refractory metal body, comprising a substrate selected from the group consisting of columbium and its al loys and having a defect, spalling, oxidation, fissuring, and thermal and mechanical shock resistant protective coating consisting essentially of, after exposure to a diffusing temperature in an oxidizing environment, an exterior exposed layer or zone composed predominantly of oxidic chromium, a pellicular subsurface layer beneath the exterior or exposed layer or zone composed predominantly of chromium, a second subsurface zone beneath the subsurface layer or subsurface zone composed predominantly of columbium silicide (preferably CbSi modified by titanium, and an inner or diffusion zone beneath the second subsurface zone composed predominantly of subsilicides of the substrate; the body being characterized by good resistance to oxidation at elevated temperatures.

In accordance with its purpose, this invention includes a method of producing a coated metal article having resistance to oxidation at high temperatures, which method comprises depositing a surface coating or shell on a metal sub strate, the substrate being a metal selected from the group consisting of columbium and alloys thereof, and the coating having a surface zone consisting essentially of a pellicular deposit of chromium and an inner zone consisting essentially of columbium silicide (preferably CbSi modified by titanium, the coating being characterized by great resistance to failure by fissuring and freedom from the silicide pest problem or failure induced by rapid oxidation at a temperature of about 1300 F.

The invention also includes a method of producing such a metal article in which the titanium-modified columbium silicide is further modified by molybdenum.

One embodiment of such method includes a two-cycle vapor deposition process of coating a fabricated base metal which process comprises surrounding a base metal selected from the group consisting of columbium and its alloys with a powdered pack of a finely ground source of silicon, a finely ground source of a metal modifier selected from the group consisting of titanium and mixtures of titanium and molybdenum, and a small amount of a volatilizable halide salt as active ingredients and an inert filler, heating the base metal and powdered pack for a time period sulficient to cause volatilization of the halide salt and to pro duce codeposition of silicon, and the metal additive on the surface of the base metal, then surrounding the deposition coated base metal with a powdered pack of a finely ground source of chromium and a small amount of a volatilizable halide salt as active ingredients and an inert filler, heating the base metal for a time period sufiicient to cause volatilization of the halogen in the halide salt and to effect the creation of an exterior surface layer or zone on the base metal consisting essentially of chromium and an intermediate layer or zone beneath the chromium consisting essentially of columbium silicide (preferably CbSi modified by the metal modifier.

When mixtures of titanium powder and molybdenum powder are used as the metal modifier an atomic ratio of titanium to molybdenum of 3:1 (weight ratio of 3:2) is preferred. But the desired beneficial results of modification can be achieved with atomic ratios of titanium to molybdenum that vary from 1:1 to 10:3 and weight ratios that vary from 1:2 to 5:3.

Such method may be extended to include the step of exposing a metal article (the exposure normally being effected during actual use), previously coated as described With columbium silicide (preferably CbSi modified by the metal modifier and a chromium overcoat, to an oxidizing environment at a diffusing temperature to effect interdiffusion between the coating and base metal and the crea tion of an outer exposed layer or surface zone consisting essentially of oxidic chromium, a subsurface layer or first subzone consisting essentially of a pellicular deposit of chromium, an intermediate layer or second subzone consisting essentially of columbium silicide (preferably CbSi modified by the metal modifier, and an inner layer or third subzone consisting essentially of subsilicides of the base metal.

As previously set forth, conventional silicide coatings on structural columbium alloy substrates are prone to rapid consumption through oxidation at low (about 1300" F.) temperatures (this tendency is sometimes referred to as the pest phenomenon) and at high (about 2000 F.) temperatures. At the latter temperature a rapid oxidation mechanism begins to occur which, though different from the pest phenomenon, is similar in its undesirable end result.

Quite unexpectedly, and contrary to what one would expect from the usual behavior of silicide coatings, if the chromium overcoated Tiand Mo-Ti-modified columbium silicide coatings of this invention are used on columbium and columbium alloy substrates, the deleterious effects of both the low" temperature pest phenomenon and high temperature rapid oxidation mechanism are essentially overcome.

The coatings of the present invention are particularly efficacious in significantly improving the reliability of prior coatings by virtually completely getting rid of the silicide pest phenomenon and failures due to fissuring. These coatings have proven their ability to protect columbium and its alloys from oxidation under a wide variety of conditions of use for substantial periods of time at temperatures up to at least 2000 F.

In accordance with this invention, typical substrates, in addition to unalloyed columbium, to which the coatings of this invention have been applied, parts expressed as percentages by weight, are as follows:

ALLOY 1 Columbium 99 Zirconium 1 (Cb-lZr) ALLOY 2 Columbium 95 Titanium (Cb-5Ti) ALLOY 3 Columbium 88 Titanium 8 Molybdenum 4 (Cb-8Ti-4Mo) ALLOY 4 Columbium 88 Titanium 8 Vanadium 4 (Cb-8Ti-4V) ALLOY 5 Columbium 93 Vanadium 5 Aluminum 2 (Cb-5V-2Al) ALLOY 6 Columbium 95 Zirconium 5 (Cb-SZr) ALLOY 7 Columbium 78 Titanium 8 Molbydenum 4 Tungsten 10 (Cb-8Ti-4Mo-10W) ALLOY 8 Columbium 94.5 Vanadium 5 Chromium 0.5

(Cb-SV-(XSCr) ALLOY 9' Columbium 87 Vanadium 7 Aluminum 3 Tungsten 3 In accordance with this invention, suitable halide salts for deposition of the metal additive are as follows:

KF ZHF CuCl 0 01 The preferred halide salt for best results in depositing the metal additive is, however, aluminum fluoride (A11 Also in accordance with this invention, suitable halide salts for deposition of chromium are as follows:

AlFg CIC1 CQCIG 2 The preferred halide salt for best results in depositing chromium is also aluminum trifluoride (AlF For a clearer understanding of the invention, specific examples of the invention are set forth in this specification. These examples are merely illustrative and are not to be understood as limiting the scope and underlying principles of the invention.

In the embodiments forming the examples of this invention an alloy consisting essentially of Cb-lZr by weight, hereafter referred to as the Alloy, was selected as a representative substrate material. Other columbium-base alloys such as those set forth previously could also have been used equally well as substrates to illustrate the new and desirable performance of the coatings for columbium base alloys described in this specification.

EXAMPLE 1 Columbium silicide coatings modified by a metal additive consisting essentially of titanium and molybdenum were applied to the Cb-lZr substrate, or the Alloy, by utilizing a pack-cementation process, which comprised embedding chemically cleaned specimens to be coated in a pack of the following mixture:

11 grams cc.) of A1 0 powder (micron particle size),

9 grams of titanium powder (minus mesh),

6 grams of molybdenum powder (minus 325 mesh),

14 grams of silicon powder (minus 200 mesh),

4 grams of AIF;,.

The packs, contained in Inconel containers, were subjected to purging by argon fiowthrough for 2 hours and then to thermal treatments at temperatures ranging from 1800 to 2200 F. and for times from about 4 to 16 hours,

with 16 hours at 1800 F. being the preferred thermal treament.

The thickness of the coating deposited Was controlled by the time temperature relationships used. A lower coating temperature is generally preferred for economy of operation, and this is particularly true when the process is scaled up. It is thus one of the advantages of this invention that its base coatings can be achieved at temperatures of about 1800' F.

In accordance with the invention, during thermal treatment an argon flowthrough was maintained in the pack at atmospheric pressure or slightly above. This flowthrough was equivalent to about 10 cc. of argon per minute through a 500 cc. canister.

When the reaction had proceeded to the extent desired, the canister was removed from the furnace and cooled under a flow of argon.

During this treatment, the silicon, titanium, molybdenum, and substrate reacted to yield a molybdenumtitanium-modified CbSi structure.

Although the exact mechanism of the modification or change wrought by Mo and Ti on CbSi in this invention is not known, it is believed that Ti and Mo atoms, respectively, are substituted for Cb atoms to a small but important extent in the CbSi lattice structure and that such substitution probably conforms to the molecular structure of TiSi and MoSi respectively.

Even though the amount of Ti and Mo substitution for Cb that takes place in the CbSi structure is small, this amount is highly effective.

The Ti and Mo modification greatly improves the reliability of the CbSi as a coating and significantly increases its plasticity giving the coating measurable ductility at use temperatures in excess of 1500 F.

Thicknesses of the molybdenum-titanium-modified silicide coating varied from 1 /2 to 4 mils depending upon specific deposition conditions. Desired thicknesses of these coatings were from about 1 /2 to 3 mils, preferably about 2 mils.

The inert filler material is not limited to A1 0 since almost any refractory oxide filler, such as zirconia or beryllia, also works well.

Although many of the halide salts will produce the desired results, ammonium halides, such as ammonium fluoride (NH F) should not be used, since they tend to cause hydrogen and nitrogen embrittlement of the columbium. Aluminum fluoride (AlF is the preferred salt, and it Works especially well because of its high vapor pressure.

The second coating cycle consisted of embedding the Alloy substrates previously deposited with silicon and the metal additive in chromizing packs containing from to 20 volume percent chromium powder, and preferably about volume percent chromium powder, AlF powder, and the balance essentially A1 0 powder.

In this example the second coating cycle comprised embedding the Alloy substrate with its deposit of silicon and the metal additive in a chromizing pack of the following mixtures:

11 grams 90 cc.) of A1 0 powder (micron particle size), 50 grams of chromium powder (minus 200 mesh), 4 grams of aluminum fluoride (AlF powder.

These packs, contained in Inconel containers, were purged with argon for 2 hours, and then subjected to thermal exposure in an argon atmosphere at temperatures ranging from 1800 to 2000 F. and for times from about 4 to 16 hours, with 16 hours at 1950 F. being used in this example. This pack cementation step resulted in a pellicular deposit of chromium over the substrate previously deposited with silicon and the metal additive. Thicknesses of the chromium coatings are less than 1 mil and typically comprise from 0.2 to 0.6 mil. In this example the thickness of the chromium overcoat was 0.5 mil.

In accordance with the invention, during thermal treatment an argon flowthrough was maintained in the pack at atmospheric pressure or slightly above. This flowthrough was equivalent to about 10 ccs. of argon per minute through a 500-cc. canister.

When the reaction had proceeded to the extent desired, the canister was removed from the furnace and cooled under a flow of argon.

Inert filler materials other than A1 0 are also suitable for use in this second pack cementation step, and almost any refractory oxide filler, such as zirconia or beryllia, will also work well.

The coating thus produced was found to protect the Alloy for periods in excess of 5000 hours at temperatures up to at least 1800 F. The coating is also protective of the Alloy for shorter periods at even higher temperatures up to 2300 F.

EXAMPLE 2 A chemically cleaned specimen of the Alloy was embedded in a pack of the following mixture:

11 grams (90 cc.) of A1 0 powder (micron particle size), 7 grams of titanium powder (minus 100 mesh),

8 grams of molybdenum powder (minus 325 mesh),

grams of silicon powder (minus 200 mesh),

4 grams of potassium fluoride (KF).

The pack contained in an Inconel container, was purged for 2 hours with argon, and then subjected to thermal exposure in an argon atmosphere at a temperature of 1800 F. for 1 6 hours. After exposure, the pack was removed from the furnace and allowed to cool under a flow of argon. The resulting coating was about 3 mils thick and consisted essentially of a molybdenum-titaniurn-modified CbSi structure.

The second coating cycle for this example consisted of embedding the Alloy substrate with its deposited silicon and metal additive in a chromizing pack of the following mixture:

11 grams (90 cc.) of A1 0 powder (micron particle size), 40 grams of chromium powder (minus 200 mesh), 8 grams of chromium chloride (CrCl powder.

The pack, contained in an Inconel container, was purged with argon for 2 hours, and then subjected to thermal exposure in an argon atmosphere at a temperature of 2000 F. for 16 hours. This pack cementation step resulted in a pellicular deposit of chromium over the substrate previously deposited with silicon and the metal additive. Thickness of the chromium coating of this example was 0. 6 mil.

In other respects, this example was prepared as set forth in Example 1 above. The coating produced by this example was found to protect the Alloy for periods in excess of 5000 hours at temperatures up to at least 1800 F. This coating is also protective over the Alloy for shorter periods at even higher temperatures up to 2300 F.

EXAMPLE 3 A chemically cleaned specimen of the Alloy was embedded in a pack of the following mixture:

11 grams cc.) of A1 0 powder (micron particle size), 15 grams of titanium powder (minus mesh),

15 grams of silicon powder (minus 200 mesh),

4 grams of aluminum fluoride (AlF powder.

The pack, contained in an Inconel container, was purged for 2 hours with argon, and then subjected to thermal exposure in an argon atmosphere of 1800 F. for 16 hours. After exposure, the pack was removed from the furnace and allowed to cool under a flow of argon. The resulting coating was about 3 mils thick and consisted essentially of a titanium-modified CbSi structure, in which titanium was present in an amount of less than 4 percent by weight of CbSi The second coating cycle of this example consisted of embedding the Alloy substrate with its titanium-modified silicide deposit in a :hromizing pack of the following mixture:

11 grams (90 cc.) of A1 0 powder (micron particle size), 50 grams of chromium powder (minus 200' mesh), 4 grams of aluminum fluoride (AlF powder.

The pack, contained in an Inconel container, was purged with argon for 2 hours, and then subjected to thermal ex posure in an argon atmosphere at a temperature of 1950" F. for 16 hours. This pack cementation step resulted in a thin deposit of chromium over the substrate previously deposited with titanium-modified CbSi The thickness of the chromium overcoat in this example was 0.5 mil.

Analysis of the foregoing examples as set forth above revealed columbium base substrates having coatings consisting essentially of Ti-modified and Mo-Ti-modified CbSi under a pellicular chromium overcoat.

FIG. 1 is a photomicograph magnified 500 times showing a representative chromium overcoated titanium-modified columbium silicide coating of this invention over the Alloy in the as-coated condition. This coating, as is characteristic of the coatings of this invention, is very uniform both in composition and thickness, well-bonded to the sub strate, and highly resistant to spalling and to failure through fissuring.

An incipient diffusion zone is also shown in FIG. 1. This zone was probably created both during the latter stages of the Ti-Si pack-cementation step and during the chromizing step.

FIG. 2 is a photomicrograph enlarged 500 times showing the chromium overcoated titanium-modified columbium silicide coating of FIG. 1 after exposure in static air for 1000 hours at 1800 F. As can be seen from the photomicrograph FIG. 2 the basic coating structure remains essentially unchanged in composition and thickness after exposure, but the narrow diffusion zone between the titanium-modified silicide zone and the substrate has been emphasized and given more definite structure by the additional residence at a diffusing temperature.

This diffusion zone consists essentially of subsilicides of the substrate. Its size in FIG. 2 as compared with its size in FIG. 1 indicates that once the diffusion zone is formed, there is very little loss of coating by diffusion into the substrate. During exposure to the high-temperature oxidizing 1 1 environment a relatively thick layer of oxidic chromium is formed as the outer or exposed zone of the specimen.

As is characteristic of the coatings of this invention, and in accordance with the invention, the coating shown in FIG. 2 has fissured down to the diffusion band, but the fissures do not go through the diffusion band. The coating of FIG. 2 is essentially stabilized and shows no signs of failure, even after 1000 hours at 1800" F. The fissures are about micron size, and although the mechanism is not clearly understood, it is believed that chromium from the overcoat helps to seal the fissures and prevent failure at these potential areas of defect.

Regardless of theory of operation, however, it is significant that the fissures do not break through the diffusion band, and as can be seen in FIG. 2, chromium appears to enter into the fissures and take part in this important characteristic of the invention in which fissuring is arrested and stopped well short of reaching the point of failure.

Coatings of this invention will thus remain essentially unchanged in the structure shown in FIG. 2 for additional thousands of hours of exposure to oxidation in static air at temperatures up to at least 2000 F.

FIG. 3 is a plot on a logarithmic scale showing continuous oxidation weight-gain data for the Alloy coated with the chromium overcoated titanium-modified columbium silicide coating of this invention. As shown in FIG. 3, although the continuous weight-gain data is for 100 hours only, it is apparent that the plot of weight-gain against exposure time is a parabolic function. The curves have been extrapolated past 1000 hours on the basis of the parabolic behavior exhibited. It has been established by actual exposure of a number of specimens that the coatings remain essentially stable with respect to weight-gain for at least 5000 hours at 1800 F.

In accordance with the invention, coatings consisting essentially of (l) columbium silicide (preferably CbSi modified by a metal additive selected from the group consisting of titanium and titanium plus molybdenum and (2) a chromium overcoat, are created by the two-cycle pack cementation process previously described. The desired chromium overcoat can be achieved with a pack mixture in which the volume percent of chromium powder is from 5 to 20 percent, and preferably percent.

The novel coatings of this invention for colurnbium and columbium base alloy substrates achieve an important, new and useful result. They possess distinct and unique advantages over the usual types of intermetallic protective coatings such as CbSi or CbAl Among the novel and unexpected beneficial results and advantages obtained from the coatings and methods for achieving the coatings of this invention are the following:

(1) The coatings of this invention consisting essentially of columbium silicide (preferably CbSi modified by titanium or titanium and molybdenum and overcoated chromium exhibit excellent long term oxidation resistance at temperatures up to at least 2000 F., and are superior to the existing coating materials, typified, for example, by CbSi and CbAl Such chromized coatings are also an improvement over nonchromized columbium silicide coatings modified by titanium or titanium and molybdenum, and chromizing conspicuously improves the reliability of such nonchromized coatings by virtually getting rid of the silicide pest problem.

(2) When compared with existing coating materials, such as CbSi the coatings of this invention virtually eliminate pest phenomenon failure or the low temperature (about 1300 F.) rapid oxidation failure that is characteristic of typical prior art silicide coatings, and the coatings of this invention retain this resistance to pest failure, even after repeated exposure to high temperature environments, thereby exhibiting excellent thermal stability. In contrast, even some improved and modified silicide and aluminide coatings are prone to fail when subjected to relatively mild thermal cycling between low and high temperatures.

(3) In their ability to resist failure by fissuring the coatings of this invention are outstandingly superior to the prior art.

(4) The coatings of this invention impart long term reliability to columbium base structures operating in static air. For example, when used as pipes for liquid metal in power plants or heat exchangers, the protection afforded the columbium substrates by these coatings enables such power plants and exchangers to be operated at higher and more efficient temperatures over longer periods of time.

(5 After treatment at a diffusing temperature, including treatment that may be concomitant with exposure to an oxidizing atmosphere during actual use of a coated article, the coatings of this invention achieve diffusional stability; the diffusion zone created consists essentially of subsilicides of the substrate between the modified silicide coating and the substrate and does not change in size. Loss of coating through diffusion into the substrate is prevented.

(6) Although a thermal expansion mismatch exists between the coatings of this invention and typical columbium base substrates, the coatings as-deposited are thin enough so that they take on the characteristics of the substrate and expand and contract with the substrate. There is excellent adherence of the coatings to the substrate and they are thus well-bonded and highly resistant to spalling.

(7) The coatings of this invention are characteristically highly uniform in both composition and thickness, are mechanically stress and shock resistant, and are compatible with columbium base substrates in that they form no melting phases or volatile compounds.

As used in this specification it will be understood that the expression CbSi includes those forms of columbium silicide in which the atomic ratio of columbium to silicon is in the order of 1:2.

The invention in its broader aspect is not limited to the specific details shown and described but departures may be made from such details within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its chief advantages.

What is claimed is:

1. An article of manufacture having good resistance to oxidation in air which comprises a core of metal selected from the group consisting of columbium and columbium base alloys, the article having a defect, spalling, fissuring and thermal and mechanical shock failure resistant surface zone consisting essentially of chromium and a subsurface zone consisting essentially of columbium silicides modified by titanium.

2. The invention as defined in claim 1, in which the titanium-modified columbium silicide subsurface zone is further modified by molybdenum.

3. An article having good resistance to oxidation in air at elevated temperatures, which article comprises a core of metal selected from the group consisting of columbium and columbium base alloys, the article having a defect, spalling, and thermal and mechanical shock failure resistant coating, the coating consisting essentially of a pellicular surface zone of chromium, a subsurface zone of columbium silicide modified by titanium, and, after exposure to a diffusing temperature in a nonoxidizing atmosphere, a diffusion zone beneath the subsurface zone consisting essentially of subsilicides of the metal core.

4. The invention as defined in claim 3, in which the titanium-modified columbium silicide subsurface zone is further modified by molybdenum.

5. An article of manufacture having good resistance to oxidation in air at high temperatures which comprises a core of metal selected from the group consisting of columbium and alloys thereof, the article having a thermal and mechanical shock failure resistant, defect resistant, spalling resistant, fissuring resistant, and broad range oxidation resistant shell, the shell being chaacterized, after initial exposure to an oxidizing environment at a diffusing temperature effective to create interdiffusiou between the shell and the metal core, by an exposed surface zone,

consisting essentially of oxidic chromium, a pellicular surface zone consisting essentially .of chromium, a second subsurface zone consisting essentially of columbium silicide modified by titanium, and a diffusion zone beneath the second subsurface zone consisting essentially of subsilicides of the metal core.

6. The invention as defined in claim 5, in which the titanium-modified columbium silicide second subsurface zone is further modified by molybdenum.

7. As an article of manufacture, a refractory metal body, comprising a substrate selected from the group consisting of columbium and its alloys and having an exterior exposed zone composed predominantly, after exposure to a diffusing temperature in an oxidizing environment, of oxidic chromium, a pellicular subsurface zone beneath the exterior exposed zone composed predominantly of chromium, a second subsurface zone beneath the subsurface zone composed predominantly of columbium silicide modified by titanium, and a diffusion zone beneath the second subsurface zone composed predominantly of subsilicides of the substrate; the body being characterized by good resistance to oxidation at temperatures up to at least 2000 F.

8. The invention as defined in claim 7, in which the titanium-modified columbium silicide second subsurface zone is further modified by molybdenum.

9. A coated metal body comprising a substrate selected from the group consisting of columbium and its alloys having a protective shell as least on that part of the substrate that is exposed to attack by oxygen and high temperatures, the protective shell having a pellicular surface zone consisting essentially of chromium and a subsurface zone consisting essentially of columbium silicide modified by titanium, the shell being oxidation-resistant, thermal and mechanical shock failure resistant, spalling resistant, defect resistant, and fissuring resistant at high temperatures.

10. The invention as defined in claim 9, in which the titanium-modified columbium silicide subsurface zone is further modified by molybdenum.

11. A coated metal body comprising a substrate selected from the group consisting of columbium and its alloys and having a protective shell at least on that part of the substrate that is exposed to attack by oxygen and high temperatures, the shell, after exposure to an oxidizmg environment at a diffusing temperature, having a surface zone consisting essentially of oxidic chromium, a pellicular subsurface zone beneath the surface zone consisting essentially of chromium, a second subsurface zone consisting essentially of columbium silicide modified by titanium, and a diffusion zone beneath the second subsurface zone consisting essentially of subsilicides of the substrate; the shell being oxidation-resistant, thermal and mechanical shock failure resistant, spalling resistant, defect resistant, and fissuring resistant at high temperatures.

12. The invention as defined in claim 11, in which the titanium-modified columbium silicide second subsurface zone is further modified by molybdenum.

13. The process of coating a fabricated base metal which process comprises surrounding a base metal selected from the group consisting of columbium and alloys thereof with a first powdered pack of a finely ground source of silicon, a finely ground source of a metal modifier selected from the group consisting of titanium and mixtures of titanium and molybdenum, and a small amount of a volatilizable halide salt as active ingredients and an inert filler, heating the base metal and powdered pack for a time period sufficient to cause volatilization of the halide salt and to produce codeposition of silicon and the metal additive on the surface of the base metal, then surrounding the deposition coated base metal with a second powdered pack of a finely ground source of chromium and a small amount of a volatilizable halide salt as active ingredients and an inert filler, heating the base metal for a time period sufficient to cause volatilization of the halogen in the halide salt and to effect the creation of a surface zone on the base metal consisting essentially of chromium and a subsurface zone beneath the chromium consisting essentially of columbium silicide modified by the metal modifier.

14. The invention as defined in claim 13, in which the process includes the further step of exposing the previously coated base metal to an oxidizing environment at a diffusing temperature to effect the creation of an outer exposed surface zone composed predominantly of oxidic chromium, a subsurface zone beneath the surface zone consisting essentially of chromium, and a second subsurface zone beneath the subsurface zone consisting essentially of columbium silicide modified by the metal modifier and a diffusion zone beneath the second subsurface zone consisting essentially of subsilicides of the base metal. 1

15. The process of claim 13, in which the halide salt is AlF 16. The process of claim 13, in which the volume percent of chromium in the second powdered pack is from 5 to 20 volume percent of the pack.

17. The process of claim 13, in which the volume percent of chromium in the second powdered pack is 10 volume percent of the pack.

18. The invention as defined in claim 13, in which the ratio by weight of titanium to molybdenum in the mixtures of titanium and molybdenum is from 1:1 to 5:3.

19. The invention as defined in claim 15, in which the ratio by weight of titanium to molybdenum in the mixtures of titanium and molybdenum is about 3:2.

20. The process of treating a metal from the group consisting of columbium and alloys thereof to render the surface of the metal resistant to oxidation at high temperatures, that includes, heating the metal to a temperature of from 1800 to 2200 F. in a nonoxidizing atmosphere and in surface contact with a powdered mixture of silicon, a metal modifier selected from the group consisting of titanium and mixtures of titanium and molybdenum, a halide salt and an inert refractory material, and subsequently heating the metal to a temperature of from 1 800 to 2200 F. in a nonoxidizing atmosphere with the surface of the metal in contact with a powdered mixture of chromium, an inorganic halide and an inert refractory material, to form thereby a protective surface shell on the metal having a surface zone consisting essentially of chromium and a subsurface zone consisting essentially of columbium silicide modified by the metal modifier.

21. The invention as defined in claim 20, in which the halide salt is AlF 22. The invention as defined in claim 20, in which the metal is heated from 4 to 16 hours in each heating step.

23. The invention as defined in claim 20, in which each heating step is carried out for 16 hours and the first heating step is at a temperature of 1800 F. and the second heating step is at a temperature of 2000 F.

24. A method of producing a high temperature oxidation resistant, thermal and mechanical shock failure resistant, spalling resistant, defect resistant, and fissuring resistant surface shell on a metal article formed of a substrate selected from the group consisting of columbium and columbium base alloys, the shell having a surface zone consisting essentially of chromium, and a subsurface zone consisting essentially of columbium silicide modified by a metal modifier selected from the group consisting of titanium and mixtures of titanium and molybdenum, the method comprising the steps of: enclosing the article in a pack of powdered material containing a source of silicon, a source of metal modifier, and a small amount of a volatilizable halide salt as essential active ingredients and an inert filler, heating the article in the pack to a temperature higher than that causing volatilization of the halide salt and maintaining this temperature for a discrete interval of time to effect the deposition of silicon and the metal modifier onto the surface of the article, then enclosing the treated article in a chromizing pack of powdered material containing a source of chromium and a small amount of a volatilizable halide salt as essential ingredients and an inert filler, heating the article in the pack to a temperature higher than that causing volatilization of the halide salt, and maintaining this temperature for a discrete interval of time to effect the deposition of chromium onto the outer surface of the article, thereby creating a surface shell having an outer surface consisting essentially of chromium and a subsurface consisting essentially of columbium silicide modified by a metal modifier selected from the group consisting of titanium and mixtures of titanium and molybdenum.

25. The method of claim 24, that includes the further step of exposing the metal article with its protective shell to an oxidizing environment at a dififusing temperature to efiect the creation of a surface zone on the shell composed predominantly of oxidic chromium, a subsurface zone beneath the surface zone consisting essentially of chromium, a second subsurface zone beneath the first sub surface zone consisting essentially of columbium silicide modified by a metal modifier selected from the group consisting of titanium and mixtures of titanium and molybdenum, and a diffusion zone beneath the second subsurface zone consisting essentially of subsilicides of the substrate.

References Cited UNITED STATES PATENTS 3,086,886 4/1963 Kieifer et al. ll7l07 REUBEN EPSTEIN, Primary Examiner US. Cl. X.R.

Patent No.

Signed and 3, 595, 633 Dated July 27, 1971 Inventorfs) EDWARD I leE LLEYI'GHEH, attesting Officer LA. Friedrich et .21

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 5, column 12, line 72, change "chaacterized" to "characterized".

Claim 19, column 14;, line 29, change "15" to l1rirl sealed this 11 th day of January 1972.

JR. ROBERT GOTTSCHALK Acting Commissioner of Patents USCOMM-DC 60375-P69 0 U 5 GOVERNMENT PNINYING OFFICE (969 (-36G-3Sl 

